I was playing an arena game. Somehow ended up with full board and 2 SN in hand, so I thought "Hey, cool, I can play 2 SN and I won't get singularity".

That's not what happened. The first one was playable, just as you'd expect, the usual SN effect occurs, you get the quantum and the card is discarded from your hand. On the other hand, when you try to play the second one, you do get the quantum, but the card doesn't get discarded and you get the "Your board is full" message. Clicking it multiple times doesn't give the quantum anymore, but you still get the message. If you play another card after all this, the SN you tried to play turns into a singularity. It's not a graphical glitch, because you get the "Not enough quanta" message if you try to actually play it.